Q.Describe the important steps in muscle contraction.
A nerve signal releases acetylcholine, an action potential frees calcium, calcium unmasks actin's active sites, and ATP-powered myosin heads form cross bridges that pull the actin inward — shortening the sarcomere and the muscle.
Muscle contraction is best understood as a sequence in which a nervous command is turned into the physical sliding of filaments. The steps below follow one another in order.
The signal arrives. A contraction does not start on its own. It begins with a command from the central nervous system that travels out along a motor neuron. Where the motor neuron meets the sarcolemma — the plasma membrane of the muscle fibre — is the neuromuscular junction, also called the motor-end plate. When the signal reaches this junction, it causes the release of the neurotransmitter acetylcholine.
An action potential spreads and calcium is released. Acetylcholine generates an action potential in the sarcolemma. This action potential does not stay in one spot; it spreads through the whole muscle fibre, and as it spreads it triggers the release of calcium ions into the sarcoplasm.
The active sites are exposed. In the resting fibre the active sites on actin, where myosin would attach, are masked by a subunit of troponin. The rise in calcium is the switch: calcium binds to troponin, the masking is removed, and the active sites on actin are exposed and made available to myosin.
Cross bridges form and pull. Now the two filaments can interact.
- Using energy obtained from the hydrolysis of ATP, the myosin head binds to the exposed active site on actin, forming a cross bridge.
- The myosin head then rotates, and this power stroke pulls the attached actin filament towards the centre of the A-band.
- Because the Z-line is fastened to the actin filaments, it too is dragged inward. As the Z-lines on either side move closer, the sarcomere shortens — and this shortening is the contraction itself.
A telltale sign of this process is that during contraction the I-bands are reduced while the A-bands stay the same length, because it is the overlap that changes, not the length of the filaments.
The cross-bridge cycle repeats. A single attachment gives only one small pull, so the cycle repeats. After the power stroke the myosin head releases its ADP and inorganic phosphate. A new ATP molecule then binds the head, which breaks the cross bridge; the ATP is hydrolysed again to re-energise the head, and it forms a fresh cross bridge. Each round produces a little more sliding, so continued cycling gives continued shortening.
Relaxation ends it. The muscle keeps contracting only while calcium is present. When stimulation stops, calcium is pumped back into the sarcoplasmic reticulum. The active sites are masked once more, the cross bridges can no longer form, the Z-lines return to their original positions, and the fibre relaxes.
In short, a neural signal and acetylcholine trigger an action potential that releases calcium; calcium exposes actin's active sites; ATP-powered myosin heads then form cross bridges and pull the actin filaments inward, and repeated cross-bridge cycling shortens the sarcomere and produces contraction until calcium is withdrawn and the muscle relaxes.
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